Based on episodes 23 and 24 of the anime and therefore containing spoilers for the entire 24 episodes.

When they were young, the rage inside him burned bright enough to scorch. He sat with his legs pulled up, his arms locked around his shins and buried his face in his knees to concentrate on not burning away.

It was Cain with his soft voice and his gentle, mocking laughter that soothed him. Cain was calm and sleek, with pale, cool hands that threaded through Abel's hair and pulled his head up to face the light. Cain with his soft voice and his languid gestures and his cool self-possession.

They would be black and white if they were not already two sides of the same reflection.

Cain appears when Abel has burnt himself out in his rage and his frustration, when he has finished striking out at anything that dares acknowledge him. Cain is there, pale and fleeting gold, like sunlight on a winter's morning, to hold Abel in his arms and wipe away the blood that stains the other boy's hands.

Abel used to believe they were fire and ice, and he used to beat at Cain's chest and dig his fingers into the other boy's flesh hard enough that it must have, couldn't have not, hurt. And Cain would simply smile and embrace him until Abel could breathe again without choking. On those days it was sometimes possible to feel something other than the flames that burnt in his veins, but only because Cain was there to dampen the heat of his wrath with his stillness. And it was but a breath in and out after Cain had left that the fire returned.

That was before the final act. Before the breaking of his wrath upon the people of the new world. Before the blood and the screaming and the minus element and the greatest of his sins.

These days Abel is stretched out and thin, and his pulse beats but weakly in a breast that is forever burdened with the sins of the ages. These days the fire that burns within him is cold as midwinter; cold as the void they threw Cain into; cold as the Hell in which they both belong.

And now when Cain smiles at him over the body of the boy at his feet, Abel remembers the last time his brother smiled at him like that and the flames that rise up to consume him are as cold as ice.